// #include <iostream>

#include <fstream>
#include <vector>
#include <queue>

using namespace std;

ifstream cin("sate.in");
ofstream cout("sate.out");

const int INF = 20000001;
const int NMAX = 30001;
int drum[NMAX];
vector<pair<int,int>>g[NMAX];
queue<pair<int, int>>q;

void bfs(int st)
{
	drum[st] = 0;
	q.push({ 0,st });
	while (!q.empty())
	{
		pair<int, int>p = q.front();
		q.pop();
		for(auto x:g[p.second])
			if (drum[x.second] == -1)
			{
				drum[x.second] = drum[p.second] + x.first;
				q.push({ drum[x.second],x.second });
			}
	}
}

int main()
{
	int n, m, x, y;
	cin >> n >> m >> x >> y;
	for (int i = 1; i <= n; i++)
		drum[i] = -1;
	for (int i = 0; i < m; i++)
	{
		int x, y, c;
		cin >> x >> y >> c;
		g[x].push_back({ c,y });
		g[y].push_back({ -c,x });
	}
	bfs(x);
	cout << drum[y];
	return 0;
}